1. Field of the Invention
The present invention relates to an ink jet ink droplet ejecting method and apparatus.
2. Description of Related Art
In a known ink jet printer, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material, and when the flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle. However, when the flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of printing head, a plurality of ink chambers is formed by partition walls made of a piezoelectric ceramic material. Ink supply means, such as ink cartridges, are connected to first ends of the ink chambers, while at the opposite, second ends, ink ejecting nozzles (hereinafter referred to as "nozzles") are provided. The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character or a figure.
For example, as this type of ink jet printer, a drop-on-demand type ink jet printer, which ejects ink droplets, is popular because of a high ejection efficiency and a low running cost. As an example of the drop-on-demand type there is known a shear-mode type that uses a piezoelectric material, as is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051.
FIGS. 8A and 8B illustrate this shear-mode type of ink droplet ejecting apparatus 600 comprising a bottom wall 601, a top wall 602 and shear mode actuator walls 603 located therebetween. Each actuator wall 603 comprises a lower wall 607 bonded to the bottom wall 601 and polarized in the direction of arrow 611 and an upper wall 605 formed of a piezoelectric material, the upper wall 605 being bonded to the top wall 602 and polarized in the direction of arrow 609. Adjacent actuator walls 603, in a pair, define an ink chamber 613 therebetween, and next adjacent actuator walls 603, in a pair, define a space 615 that is narrower than the ink chamber 613.
A nozzle plate 617 having nozzles 618 is fixed to first ends of the ink chambers 613. An ink supply source (not shown) is connected to the opposite ends of the ink chambers. As illustrated in FIG. 8B, on both side faces of each actuator wall 603 are formed electrodes 619 and 621 respectively as metallized layers. More specifically, the electrode 619 is formed on the actuator wall 603 on the side of the ink chamber 613, while the electrode 621 is formed on the actuator wall 603 on the side of the space 615. The surface of the electrode 619 is covered with an insulating layer 630 for insulation from ink. The electrode 621 that faces the space 615 is connected to a ground 623, and the electrode 619 provided in each ink chamber 613 is connected to a controller 625 that provides an actuator drive signal to the electrode.
The controller 625 applies a voltage to the electrode 619 in each ink chamber, whereby the associated actuator walls 603 undergo a piezoelectric thickness slip deformation in different directions to increase the volume of the ink chamber 613. For example, as shown in FIG. 9, when a voltage E(v) is applied to an electrode 619c in an ink chamber 613c, electric fields are generated in directions of arrows 631 and 632 respectively in actuator walls 603e and 603f, so that the actuator walls 603e and 603f undergo a piezoelectric thickness slip deformation in different directions to increase the volume of the ink chamber 613c. At this time, the internal pressure of the ink chamber 613c, including a nozzle 618c and the vicinity thereof, decreases. The applied state of the voltage E(v) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber 613. During this period, ink is supplied from the ink supply source.
The one-way propagation time T is a time required for the pressure wave in the ink chamber 613 to propagate longitudinally through the same chamber. Given that the length of the ink chamber 613 is L and the velocity of sound in the ink present in the ink chamber 613 is a, the time T is determined to be T=L/a.
According to pressure wave propagation theory, upon lapse of time T or an odd-multiple time thereof after the above-application of voltage, the internal pressure of the ink chamber 613 reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode 619c in the ink chamber 613c is returned to 0(v). As a result, the actuator walls 603e and 603f revert to their original state (FIG. 8A) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure and the pressure developed by reverting of the actuator walls 603e and 603f to their original state before the deformation are added together to afford a relatively high pressure in the vicinity of the nozzle 618c in the ink chamber 613c, whereby an ink droplet is ejected from the nozzle 618c. An ink supply passage 626 communicating with the ink chamber 613 is formed by members 627 and 628.
Conventionally, in this kind of apparatus 600 for jetting droplets of ink, when a printing frequency requires an increase of when droplets of ink of consecutive dots are jetted then, within a certain frequency range, the ink-jet tends to become unstable due to a meniscus vibration of ink within the nozzle. As a consequence, during continuous ink-jetting, jet speeds of second and third ink droplets and volumes of ink droplets are fluctuated and become uneven, thereby resulting in decreased printing quality.
Conventionally, as shown in Japanese Published Unexamined Patent Application No. Hei 6-84073, to compensate for the influence of the meniscus vibration of ink-jetting and to effectively use energy required when a pulse voltage rises, there is a method known in which a time period ranging from the trailing edge of a pulse voltage to the leading edge of the next pulse voltage is set to 1/2 of a natural vibration period of a nozzle portion. However, according to this method, vibration of the next ink-jetting is overlapped with vibration generated when a piezoelectric element returns to a stable position after a vibration of ink-jetting is stopped. This method does not provide a counter-measure executed during the continuous vibration at a high printing frequency.
Additionally, as shown in Japanese Published Unexamined Patent Application No. Sho 61-120764, a method is known in which a drive signal for a piezoelectric element is controlled with reference to a dot interval in such a manner that the volume of droplets of ink remains constant regardless of the dot interval. However, this method is not able to prevent fluctuation of the volume of ink droplets of a second and subsequent continuous dots.